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Aldoses exist almost exclusively as their cyclic hemiacetals very little of the open chain form is present at equilibrium To understand their structures and chemical reac tions we need to be able to translate Fischer projections of carbohydrates into their cyclic hemiacetal forms Consider first cyclic hemiacetal formation m d erythrose To visualize furanose nng formation more clearly redraw the Fischer projection m a form more suited to cyclization being careful to maintain the stereochemistry at each chirality center... 

The Fischer projection is a convenient way of showing the configurations of the linear forms of monosaccharides. This convention depicts the concepts of stereochemistry established by Jacobus Henricus van t Hoff and Joseph Achille Le Bel in a simplified form. While these abbreviated structural formulas are simple to write and easy to visualize, there are some guidelines that should be taken into account when converting a three-dimensional structure into a Fischer projection and in its manipulation (Fig. 1.2) ... 

A major drawback of cyclic Fischer projections is the unrealistic manner in which the structures are depicted. In 1929, Haworth designed a representation to address this deficiency. Haworth projections provide a simple way to represent cyclic monosaccharides with a three-dimensional perspective. The following process allows the conversion of a Fischer projection into a Haworth representation ... 

Do not rotate a Fischer projection formula in the plane of the page, because you might inadvertently convert a compound into its enantiomer. When using Fischer projections it is usually best to convert them to structures with wedges and dashes, and then manipulate them. Although a Fischer projection formula can be used for the stereogenic center in any compound, it is most commonly used for monosaccharides. 

In a Fischer projection, the bonds to the central carbon are represented by horizontal and vertical lines from the substituent atoms to the carbon atom, which is assumed to be at the center of the cross. By convention, the horizontal bonds are assumed to project out of the page toward the viewer, whereas the vertical bonds are assumed to project into the page away from the viewer. The Glossary of Compounds found at the back of the book is a structural glossary of the... 

The Fischer Projection is a two-dimensional drawing of a molecule that shows a chiral carbon at the intersection of two lines. Horizontal lines represent bonds projecting out of the page and vertical lines represent bonds that project into the page. The most oxidized carbon is always represented at the "top" of the structure. 

In the Fischer Convention, the type of structural formula used is the Fischer projection. Thus we should convert the structural representation of the glyceraldehydes given in the problem into the Fischer projection. First, a cross is drawn. The location of the chiral (assymmetric) carbon is represented by the intersecting point of the two lines of the cross. The horizontal lines extending to the left and right of this point represent bonds extending forward, above the plane of the paper. The vertical lines represent bonds extending back, below the plane of the paper. 

Three pairwise switches turn the first Fischer projection into the second one. The odd number of switches means that the structures were enantiomers of each other. 

Fischer projection /fish-er/ A way of representing the three-dimensional structure of a molecule in two dimensions. The molecule is drawn using vertical and horizontal lines. Horizontal lines represent bonds that come out of the paper. Vertical lines represent bonds that go into the paper (or are in the plane of the paper). Named for Emil Fischer, the convention was formerly used for representing the absolute configuration of sugars. 

Monosaccharides have chiral centers and thus exhibit optical activity. Let s explore the structure of glyceraldehyde. In the three-dimensional representation, the dotted wedges indicate the bonds that extent backwards from the chiral carbon (away from you or into the plane of the page) and the solid wedges indicate the bonds that are projected toward you (out of the plane of the page). In stereochemistry, Fischer projection is an important way to represent the spatial orientation of molecules. In Fischer projection representation, the bonds that are pointed backward (away from you or into the page) are indicated by vertical lines, and the bonds that extend toward you (out of the page) are represented by horizontal lines. For a more detailed approach, refer to Chapter 19 (Stereochemistry). 

The answer is D. The given structure is an ethane derivative, not a methane derivative. This rules out Choices A and B. In the Fischer projection, the horizontal lines represent the bonds extending out of the plane of the paper toward you. The vertical lines represent the bonds pointing into the plane of the paper (away from you). With this in mind, we can start the switching of groups as follows ... 

We know that bonds do not go around corners, so let s see how we convert a Fischer projection into a chair structure. There are many ways to do this task, and any way you devise will do as well as the following method. Just be certain that your method works We will continue to use D-glucose as an example and work through a method of arriving at a three-dimensional structure. 

PROBLEM 22.11 Transform the Fischer projection of D-mannose (Rg. 22.8) into the chair structure of P-D-mannopyranose using the method described in Rgures 22.17 and 22.19. Confirm your answer by using the fact that D-mannose is the C(2) epimer of D-glucose. 

A Fischer (Real Life 5-2) projection is a simphfied way of depicting tetrahedral carbon atoms and their substituents in two dimensions. With this method, the molecule is drawn in the form of a cross, the central carbon being at the point of intersection. The horizontal lines signify bonds directed toward the viewer the vertical lines are pointing away. Hashed-wedged line structures have to be arranged in this way to facilitate their conversion into Fischer projections. 

Conversion of the Hashed-Wedged Line Structures of 2-Bromobutane into Fischer Projections (of the Stereocenter)... 

Conversion of Hashed-Wedged Line Structures into Fischer Projections... 

We now have a simple way of establishing whether two different Fischer projections depict the same or opposite configurations. If the conversion of one structure into another takes an even number of exchanges, the structures are identical. If it requires an odd number of such moves, the structures are mirror images of each other. 

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